Unfortunately, I couldn't find much time for this activity, but as far as I understand, thread [1] only requires jsonpath documentation right now. So I extracted the relevant parts from this patch, reworked path expression description, and moved it to func.sgml as Peter suggested (attached). Nikita is going to add this patch to the jsonpath thread together with the updated code once it's ready.

Next, I'm going to address Peter's feedback on the rest of this documentation patch (which probably also needs to be split for threads [2] and [3]).